Membrane-based separation is a rapidly growing green technology that has been commercially recognized in many industrial applications. Most commercial asymmetric membranes are fabricated from engineering plastics using different techniques to obtain ideally ultra-thin skin layer having a high microporosity with molecular sieve characteristics. Additional requirements for membrane materials include ability to form flexible membranes, free of large defects, improved damage tolerance, stable and durable under operating conditions. Fulfillment of all these requirements is hard to achieve and improvement of membrane materials is still an important topic of research due to the trade off between permeability and selectivity that is usually a challenge especially in some applications such as separation of oxygen from air [1].
Generally, most of the polymers require incorporation of additives, whether inorganic or organic, to improve the processability, performance, durability and desired characteristics or properties which could not be obtained by only one pure polymer. Interpenetrating polymer networks (IPNs) were introduced in the early 1960s and led to a revolutionary success in creating polymeric nano-scale blends having new extraordinary properties as reviewed elsewhere [2,3]. The importance of IPN synthesis has been recognized through huge numbers of engineering literature, patents and commercialized products reported since 1951 [4]. The concept of in situ polymerization within and through structure of another polymer network as well as stabilizing this multi-phase system was based on creating interpenetrating (i.e. physical interlocked or/and catenated) networks having multi-domains at supramolecular levels. The concept of synthesis of IPNs seems to be useful for development of nano-size multiphase polymeric materials for gas separation membranes as it provides a distinct possibility to control composite material properties and morphology. However, in situ structuring and interfacial tailoring of IPNs is necessary to obtain desired properties and to overcome challenges such as defects in the fine structure [5], phase separation and incompatibility [6,7].
There are several routes for synthesis of IPNs with large topological variations. The chemical compositions of these IPNs can be selected from a variety of monomers, oligomers, prepolymers, polymers, crosslinkers and initiators as reported elsewhere [3,4]. It is pointed out that each specific application, requires a selection of the appropriate type of IPN, chemical composition and in situ synthetic procedures and processing including shaping and post-treatment of the final end-use products. For example, semi-IPN type polymer alloy was synthesized as a porous material to prepare microporous membranes as disclosed in a US patent [8]. This patent combines two known techniques of IPNs synthesis [3,4] and membrane formation [9] to introduce hydrophilic property into traditional asymmetric hydrophobic membranes. This patent reports using mono-functional vinyl monomer or/and oligomer containing a hydrophilic moiety and then making a strong connection for this monomer or/and oligomer to the hydrophobic non-crosslinked polymer through other multifunctional monomer or/and oligomer free of hydrophilic structure moiety. The advantages of hydrophilic surface of membranes are to improve the flux of water or other non-hydrophobic solvents as well as to make membranes fouling-resistant to hydrophobic solutes such as proteins. The application of the ultrafiltration membranes requires membranes with an average pore size diameter of 5.2 to 6 nm to separate more than 90% of Bovine Serum Albumin (BSA) solute that has a Stokes diameter of 7.74 nm as reported elsewhere [10]. The hydrophilic nature of these membranes and the relatively larger pore size than the molecular size of industrial gases such as nitrogen or oxygen makes these membranes not suitable for gas separation. It is also disclosed that a major disadvantage of their invention was that a very small flux was obtained for a low molecular weight cut-off membrane. It also appears that their route of IPN synthesis and formation of asymmetric membranes is unsuitable to prepare highly microporous gas separation membranes for the following reasons:
One of the ingredients of the casting solution in their invention could be alcohol such as ethanol, which acted as a solvent for non-crosslinked polymer.
In their invention, polymerization and gelation of the crosslinkable vinyl monomers or oligomers was accomplished after casting the film and subsequent irradiation by UV light, which was followed by coagulation in water. The presence of gel particles within the non-crosslinkable polymer solution may lead to a heterogeneous cast film that influences the dynamic of phase inversion process and asymmetric membrane morphology.
In their invention polymerization and gelation occurs in the absence of mixing.
In their invention, use of vinyl monomers requires initiators that results in additional cost, safety issues and contamination that may create large defects in the thin skin layer essential for membranes suitable for gaseous separations.
Other examples can be found in literature to prepare similar semi-IPN. In one of those examples, BMI/polysulfone semi-IPN was prepared from a casting solution of BMI, polysulfone and anionic initiator (1,4-diazabicyclo-[2,2,2]-octane) in N-methylpyrrolidone solvent. Thermal polymerization of BMI was achieved to gel the cast film. However, the morphology of the cured films examined by optical microscopy (magnification 1200 times) showed a phase separation [11]. It was attributed to heating the stagnant cast film at a high curing temperatures over glass transition temperature of PSF before and during polymerization of BMI that leads to this phase separation as illustrated elsewhere [12,13]. However, an improvement in the synthesis of semi-IPN was achieved in the work of Liou and coworkers [14] when BMI oligomer/polyimide was mixed in a rotary roller for 24 hours within a high viscous NMP/polyimide solution. BMI oligomers act as a plasticizer for polyimides and leads to formation of semi-IPNs containing microphase domains that are smaller than 0.25 micrometer, which is beyond the resolution limit of optical polarized microscopy [14]. However, the work was aimed to prepare semi-IPNs suitable for microelectronic industry.